This invention relates to a carburetor of an internal combustion engine, and more particularly relates to a variable venturi type carburetor in which, during the no-load running operation or idle driving condition the atomization characteristic is improved and the change in the amount of fuel being fed to the engine combustion chambers is reduced.
A variable venturi type carburetor comprises a suction piston movably mounted on a venturi portion of an intake passage so as to vary the cross-sectional area thereof in accordance with the change in the amount of air being fed to the engine combustion chambers. As is well-known, the cross-sectional area of the venturi portion is controlled so that the velocity of air flowing in the venturi, that is, the vacuum level in the venturi, is always maintained at a constant value. A tapered needle fixed to the suction piston enters into a fuel injection jet communicated with a fuel float chamber so that an amount of fuel proportional to the amount of air flowing in the venturi is sucked into the intake passage.
From the practical point of view, however, the velocity of air flowing in the venturi, or the vacuum level therein is not completely constant at any time, but shows a tendency to increase when the amount of air flowing in the venturi increases. Therefore, if a variable venturi type carburetor is set so as to attain the most suitable velocity or vacuum level of air when the amount of suction air is maximum, a sufficient velocity or vacuum of air flowing in the venturi cannot be obtained when the amount of suction air is small, that is when the engine is in the no-load running operation or the idle driving condition. Thus, during the idle driving condition, very fine grained fuel cannot be injected from the fuel injection jet. That is, the atomization characteristic is lowered, because the velocity of air flowing in the venturi is lowered and a sufficient vacuum therein cannot be obtained. Furthermore, during the idle driving condition, the vacuum level in the venturi is greatly influenced by the vibration of the different parts of the carburetor, such as the venturi portion and the suction piston, caused by the vibration of whole engine, because the cross-sectional area of the venturi is small. That is to say, the velocity of air flowing in the venturi and the vacuum level therein are changed, so that the amount of suction air is also largely changed. Therefore, during the no-load running operation or idle driving condition, a sufficient uniform air-fuel mixture cannot be obtained. Especially, in a multi-cylinder type internal combustion engine, the fuel distribution into each cylinder cannot be performed uniformly, the fuel combustion in the engine cannot be stabilized and misfires may occur, so that smooth operation of the engine is disturbed. In addition, fuel consumption during the idle driving condition is not economical and noxious combustible substances, such as hydrocarbons (HC) or carbon monoxide (CO), are contained in the exhaust gas.